In general, an automatic transmission mounted in a vehicle includes a plurality of planetary gear sets (planetary gear mechanisms), and a plurality of hydraulic frictional coupling elements such as clutches and brakes. These frictional coupling elements are selectively engaged by hydraulic control to change power transmission paths through the planetary gear sets, thereby providing a plurality of forward gears and usually one reverse gear.
For example, PATENT DOCUMENT 1 discloses an automatic transmission, which includes three planetary gear sets of a single pinion type, and five frictional coupling elements. Two of the frictional coupling elements are engaged to provide one of six forward gears and one reverse gear.
In recent years, there is a growing demand for an increase in the number of forward gears to improve the fuel efficiency of an engine and transmission performance. For example, an automatic transmission suggested includes three planetary gear sets and six frictional coupling elements, and provides eight forward gears by combining two of these frictional coupling elements.
In this configuration, however, four of the frictional coupling elements are disengaged in each gear stage. The sliding friction between the friction plates of the disengaged frictional coupling elements or the viscous resistance of the lubricant between the friction plates or any other factor increases the drive loss of the automatic transmission as a whole. This may hinder an improvement in the fuel efficiency, which is the advantage in increasing the number of gears.
PATENT DOCUMENT 2 discloses an automatic transmission including two planetary gear sets of a single pinion type, two planetary gear sets of a double pinion type, and five frictional coupling elements. Three of the frictional coupling elements are selectively engaged to provide eight forward gears.
In this configuration, two of the frictional coupling elements are disengaged in each gear stage, thereby reducing the above-described drive loss. In addition, the planetary gear sets of the double pinion type are also used, which are more flexible than planetary gear sets of the single pinion type in setting gear ratios. As a result, proper setting of a gear step between two successive gears can be expected. The gear step is obtained by dividing the gear ratio in an n-th gear (where n is an integer) by the gear ratio in the (n+1)-th gear.